The result is a fuller, richer sound—the listening difference, roughly, between a transistor radio and stereo headphones. When using earbuds during a call, the sound improvement can hardly be overstated. And even over speakerphone, the technology's noise-cancellation capabilities are significant. A person in a busy restaurant, or walking on a windy day, can sound as though he's in a quiet room.

T-Mobile announced the launch of HD Voice on its nationwide network in January 2013, ahead of its "Un-Carrier" moves and major subscriber additions. To experience it, however, both parties have to be on an HD Voice-capable phone.

Sprint CEO Dan Hesse confirmed March 25, during a speech at the Oracle Industry Connect conference in Boston, that Sprint will launch HD Voice nationwide in July. During the carrier's Feb. 11 earnings call, executives had put the rollout data at "midyear," and said that 12 million customers already had HD Voice-enabled smartphones.

"Voice is still the killer app," Hesse said, according to a report from Light Reading, which added that Hesse offered an explanation of how the spectrum Sprint gained through its acquisition of Clearwire is making HD Voice and its high-speed Sprint Spark initiative possible.

Spectrum concerns have been a primary reason carriers have held back. But to date, 100 mobile operators worldwide have launched HD Voice services, the Global Mobile Suppliers Association said in a March 25 report, marking the milestone. According to the report, 1 in 5 HSPA operators now offer HD Voice, and 19 manufacturers now make a total of more than 329 HD Voice-enabled phones.

AT&T introduced the Asus PadFone X, a smartphone-tablet duo, at the Consumer Electronics Show in January, saying that it would be the carrier's first HD Voice device. In early 2013, AT&T promised HD Voice for late 2013 and then delayed it to sometime in 2014, the same timeframe Verizon Wireless has given.

AT&T had been mum about the PadFone X, until on March 27, when it announced the device's specs—Android 4.4 (KitKat); a 5-inch display on the phone and a 9-inch display on the tablet; a 13-megapixel rear camera on the phone—though still not its launch date.

While global mobile voice service revenue continues to decline, voice use is actually rising.

"Recent results from Vodafone reveal that while its European mobile messaging traffic declined by 23 percent in a year, its voice traffic actually increased by almost 4 percent," Yankee Group Vice President of Research Declan Lonergan said in a March 26 blog post.

"Meanwhile, the world's largest OTT communications app, WhatsApp, is planning to add voice capability during Q2 2014. This month, its rival LINE added low-cost voice calling in eight markets," Lonergan continued. "… Voice continues to be an important application for end-users—irrespective of whether it's delivered over IP (VoIP) or as a cellular-based service. Operators should continue to invest in their networks to ensure they maintain strong capability and consistent performance."